The present disclosure relates to a device and method for detecting an emergency condition for shutting down a gas turbine engine, and particular to a device and method that detects a shaft failure of the gas turbine engine.
Gas turbine engines have been well known in the art for many years, and are engines in which a shaft is used to transmit the torque delivered by a turbine assembly to a compressor assembly. The compressor is used to pump a working fluid (typically air) through the engine, a combustion system (located between the compressor and turbine) is used to add thermal energy to the working fluid, the turbine assembly is used to extract work from the working fluid to drive the compression system, and the residual energy in the working fluid is used to provide shaft power (turboshaft, or turboprop) or motive thrust (turbojet or turbofan).
Engines have been created which incorporate from one to any number of shafts, typically designated as single, dual, triple spool or multi-spool engines. Multi-spool engine designs will generally include two or three coaxial drive shafts. In a dual spool engine, a low pressure compressor is connected by a first coaxial drive shaft to a low pressure drive turbine. Downstream of the low pressure compressor assembly is a high pressure compressor which is connected to a second coaxial drive shaft which is driven by a high pressure turbine assembly.
A similar arrangement is provided in a three spool engine design except that there is now a low, intermediate, and high pressure compressor assembly that are each connected respectively to low, intermediate, and high pressure turbine assemblies. In some configurations, a final turbine assembly may be used to drive an external load rather than driving a second or third compressor assembly, in which case the turbine assembly is often referred to as the power turbine.
Whether the engine is the single, dual, or triple spool type, the drive shafts must be capable of rotating at thousands of rpm's for hours at a time, with significant variations in operating temperature, acceleration demands, centrifugal stress, axial stress, etc. In extremely rare instances during the life of an engine, situations can occur where the load on a drive shaft exceeds design limits which may result in failure of the affected shaft. Failure or “decoupling” of one or more of the engine shafts will generally occur suddenly and can lead to an uncontained failure of the released turbine assembly.
When a gas turbine engine experiences a shaft failure between the compressor and turbine assemblies, the entire failure sequence may occur in less than one second, and will result in the sudden deceleration of the affected compression system, while the turbine assembly rotating components will experience an unregulated and rapid acceleration. The rapid acceleration of the turbine assembly poses the greatest hazard to the engine and vehicle because the increased rotational velocity may introduce forces on rotating assembly that exceed the mechanical strength limits of the assembly.
When rapid acceleration occurs the turbine assembly will experience a failure of the disk or blade components leading to the release of high energy debris. Most gas turbines are not designed to contain such high energy release of failed hardware due to weight and cost constraints. On a jet aircraft, an uncontained turbine disk or blade failure could result in serious damage to other engines or aircraft hardware and could result in loss of the aircraft. Aviation safety regulators mandate that in the event of a shaft break the gas turbine must not release high energy debris.
Various attempts have been made to contain a component burst through the engine housing. In one such attempt, a solid containment ring formed of high strength material, such as a nickel cobalt alloy has been integrated into the outer engine housing to circumferentially surround the rotating components of the engine. Although such containment rings have been successful in containing fragmented components within the engine housing, they add a significant amount of additional weight to the engine, thus sacrificing fuel economy and passenger capacity. It is desirable to detect and act on a shaft failure event quickly, to prevent excessive stresses on the released turbine components.
Because shaft failure indications may only be available for a short time (fractions of a second) before the turbine rotating components start to fragment, it is evident that such a warning protocol must be automated, preferably in the form of a control logic utilized by a high-speed on board processor. If a shaft failure can be detected quickly and the engine can be shut down while the engine is displaying the early warning signs of shaft failure, and before any component fragmentation occurs, the need for using heavy containment rings can be eliminated. Additional damage to the engine resulting from the high speed component fragmentation can be eliminated as well. In this fashion, the safety of the operational engine can be significantly improved, the potential for component fragmentation can be eliminated and the safety and integrity of the vehicle maintained.
As mentioned above, gas turbine engines (e.g. jet engines) include a rotating shaft having a compressor and/or a turbine assemblies mounted thereon and rotating therewith. Excessive axial movement of the shaft supporting the turbine and compressor assemblies relative to the static structures of the engine is considered to be abnormal and indicative of engine failure (e.g. shaft breakage). Detection of axial movement of the shaft relative to the remainder of the engine can therefore be used to detect engine failure and to activate a shut off of the engine. If the shaft linking the turbine to a compressor is broken, the loads on the turbine assembly will act to push it backwards (towards the engine exhaust). The compressor loads however will act to push it forward (towards the engine intake) even as the compressor rapidly decelerates. The turbine elements will increase rotational speed due to the loss of the compressor load and the continued availability of energy from the combustion system.
Previous systems for detecting turbine shaft failure have relied on continuity systems to detect abnormal movement of a turbine assembly relative to the engine casing. In those systems, when the axial movement of the turbine assembly exceeds a minimum level, the turbine assembly breaks a continuity circuit and in so doing shuts off fuel flow to the engine. In another system, movement of the circuit breaking element relative to the shaft breaks a circuit and thereby produces a signal. In a further approach, an electro-optic sensor senses unwanted or abnormal axial movement of turbine blades or rotors of a gas turbine.
Another form of a broken shaft detection system uses a detector assembly mounted downstream of a power turbine wheel of a gas turbine engine to detect rearward axial motion of the wheel indicative of a broken shaft event. The detector assembly has a plunger positioned to be axially displaced against a link connected in an electrical circuit which may be broken when the plunger is displaced thereby creating an open circuit that may be detected by a detection and test element. This detection may be communicated to an over-speed circuit that controls a shut-off switch that interrupts fuel flow to the engine. In another approach, a frangible sensor element is cut by a separating tang mounted on and moving axially with a gas turbine shaft when the shaft fails. The systems employed in the prior art thus rely on the addition of electrical and mechanical components to the engine, which increases the weight and complexity of the engine.
It is very important to avoid false shaft failure detection events. Typically a sensor monitoring for shaft breakage is directly coupled to a fuel cut-off circuit to automatically and quickly shut off the engine when the shaft breaks. A false detection would therefore lead to an unwarranted shut down of the engine which by itself increases the threat to the vehicle. For systems relying upon electrical circuits, discrepancies in the performance of the detection circuit itself can trigger a false shaft break signal.